1. Technical Field of the Invention
The present invention relates in general to optical bench systems, and in particular to the production of monolithic optical bench systems micromachined on a substrate.
2. Description of Related Art
Micro Electro-Mechanical Systems (MEMS) refers to the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through microfabrication technology. For example, the microelectronics are typically fabricated using an integrated circuit (IC) process, while the micromechanical components are fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical components. MEMS devices are attractive candidates for use in spectroscopy, profilometry, environmental sensing, refractive index measurements (or material recognition), as well as several other sensor applications, due to their low cost, batch processing ability and compatibility with standard microelectronics. In addition, the small size of MEMS devices facilitates the integration of such MEMS devices into mobile and hand held devices.
In optical applications, MEMS technology may be incorporated into an optical bench system to enable one or more optical elements to be moveably controlled by a MEMS actuator. Among these applications are interferometers, spectrometers, tunable optical cavities, fiber couplers, optical switches, variable optical beam shapers, optical micro scanners, variable optical attenuators, tunable lasers and many other applications in both sensor and telecommunications domains.
Conventional Silicon On Insulator (SOI) wafer optical benches produce optical elements that are flat in the direction perpendicular to the wafer surface. Although such optical elements are able to reflect, refract or diffract an impingent optical beam, the optical elements typically are not able to focus or collimate the optical beam. As a result, conventional optical bench systems suffer from weak coupling efficiency, high propagation loss and/or limited allowable optical beam travel distance inside the optical bench. These shortcomings limit the overall optical performance of such systems.
More recently, an optical bench system with collimation has been proposed using vertical flat mirrors attached to a tensional mechanical motion mechanism to produce an N×N optical MEMS switch, with the optical beam collimation function being implemented by the use of lensed optical fibers. However, such lensed optical fibers are costly and have a limited optical working distance.
Cylindrical mirrors (curved in the in-plane direction and flat in the out-of-plane direction) attached to linear MEMS actuators to focus the beam in the in-plane direction have also been proposed to produce variable optical attenuators and to increase the coupling efficiency of MEMS tunable lasers. To carry out the beam focusing in the out-of-plane direction, an HF thinned optical fiber functioning as a rod cylindrical lens was inserted. However, the addition of this fiber renders the optical system non-monolithic, in addition to the limited focusing ability due to the standard fiber circular cross section radius of curvature. Furthermore, the use of two different surfaces instead of a single surface leads to additional reflection loss and reduces the 3-D shaping capability.
Another 3-D focusing optical element has recently been introduced that uses refractive elements composed of two cylindrical surfaces tilted ±45° with respect to the substrate forming a rotation angle of 90° between them. The element may be etched using collimated X-ray beam lithography titled with angles −45° and 45° relative to the substrate to pattern the resist. However, the mirrors are cylindrical and require special lithography with special alignment, which again limits the performance in addition to being able to work in a refraction configuration only.
3-D micro optical bench systems that require further assembly steps after fabrication were also recently introduced. Rotational assembly was used to create micro optical subsystems that process free space beams travelling above the surface of a chip, where the optical elements after fabrication are rotated 90° and held from the side by latches. In these systems, the definition of the optical axis is governed by the accuracy and stability of the mechanical elements. Moreover, the optical axis lies above the substrate, preventing the monolithic integration of grooves for source insertion and rendering the integration of MEMS actuators, for moving the optical components in the in-plane or out-of-plane direction, a difficult task.
Hybrid integrated 3-D optical benches were also recently introduced. A micro device consisting of an in-plane polymer lens, a thick fiber holder and a mechanical shutter driven by an electrothermal actuator has been demonstrated by integration of polymer lens, poly-Si MUMPs and single-crystal-silicon HARM structures on a SOI wafer. Mechanical mounting systems for connecting and aligning optical components on an optical bench including focusing elements and other optical components were proposed as well. However, this hybrid integration is an obstacle for batch fabrication of monolithically integrated systems.
Attempts have been made to batch fabricate 3-D curved optical elements shapes on different surfaces, such as glass and silicon. However, most of the well-known methods produce the optical element lying on the surface, which does not enable the micro optical bench to manipulate in-plane optical beams. Reported 3-D concave structures typically work as photonic crystal mirrors reflecting out-of-plane optical beams, and can neither focus and collimate in-plane optical beams nor allow the insertion of the optical source on the wafer substrate.
Therefore, what is needed is a monolithic optical bench system containing a 3-D curved optical element capable of manipulating in-plane optical beams and that can be integrated with an optical source. In addition, what is needed is a monolithic optical bench system that includes both a 3-D curved optical element and one or more moveable optical elements that can be actuated by a MEMS actuator.